After his appearance in Dead or Alive 2, Ein was replaced by both his real identity Hayate, and Hitomi, a friend of Ein who shares the same martial arts style, karate. Despite Hayate being Ein only temporarily, this persona has become a recurring unlockable character in the rest of the Dead or Alive games following his first appearance.

Hayate was kept at DOATEC Germany and the project team managed to repair Hayate's spinal cord and bring him out of his coma (which he had entered after being seriously injured by Raidou). However, before they could continue, Ryu Hayabusa attacked the German headquarters to save Kasumi, who was being held captive in the same building.

Hayate was put on a helicopter with Lisa to be taken to another DOATEC lab, but the engine failed and the helicopter crash-landed in the Black Forest. Although wounded, Hayate got away from Lisa and was lucky enough to stumble into a clearing where Hitomi was training, mistaking her for Kasumi before collapsing.

As a result of the traumatic events, Hayate had gained extreme amnesia and couldn't even remember his own name. While he was staying with Hitomi and her father, he was named "Ein" and soon mastered karate in a short period of time.

Wanting to find out who he was and where he came from, Ein entered the second tournament on a journey of self-discovery. During the tournament, Ein met Hayabusa and Ayane, who both wondered if he was really Hayate. He also met Helena Douglas, who talked about the failed Project Epsilon, confusing Ein.

After the second tournament, Ein finally met up with Kasumi, Hayate's younger sister and, following Hayabusa's advice, they battled each other. After defeating Kasumi, Hayate finally remembered who he was.

Hayate joined the third tournament in order to find Genra and investigate DOATEC. To do this, Hayate disguised himself using his "Ein" identity. While on the Freedom Survivor, Hayate (disguised as Ein) met Hitomi again for the first time since he left her family's home when he was Ein. Hayate introduced Ayane to Hitomi, who assumed that Ayane was Kasumi, the girl who Hayate was mumbling about when they first met. Angered for being mistaken for Kasumi, Ayane fought Hitomi. After the fight, Hayate revealed to Hitomi the truth about his shinobi identity, much to her surprise.

In terms of physical appearance, Hayate doesn't look much different. He's well built, he has brown hair and light brown eyes. He also has a more relaxed expression on his face. As for his clothing, he wears alot of costumes that expose his skin; His usual attire consists of an open black shirt, with only a mesh shirt underneath, complete with jeans and boots. His other outfits include a silver shirt unbuttoned to his stomach, and a pair of red slightly unbuttoned pants, chains on the arms, and he's shirtless and barefoot. He can also wear his karate gi in combat as well.

In the latest title, Dead or Alive 5 Ultimate, his appearance was modified to give him a more realistic appearance, with smaller eyes and less anime-like facial features, as well as somewhat less muscular definition; He loses some of his abdominal and pectoral definition but still retains his toned back and arms. He sports sunglasses, and wears basically older outfits from previous installments. His new outfits still reveal some skin, such as his Incubus DLC Halloween costume, and a casual tracksuit, unzipped all the way to his navel, as well as his 2015 Halloween DLC where he wears a skimpy loincloth with skimpy trunks underneath.

Ein's personality is somewhat similar to Hayate as he seems to be reserved and quiet. However, Ein also seems ignorant at times; he would rather fight than converse to find out answers. Ein prefers to just fight than talk with his opponent.

The name "Ein" likely comes from "eins," the German word for "one" or "first." It's been noted that the letters of "Ein" are also the first, middle, and last letters of Epsilon. Whether or not this was intentional remains to be seen.

After the first tournament, Hayate was found in the Black Forest of Germany by Hitomi, whose family took him in. After being used for the Epsilon Project he suffered from amnesia, he could not remember his past or even his name and ended up going by the name of "Ein." The two of them trained together under her father but Ein soon departed for the second Dead or Alive tournament, where he recovered from his amnesia. After this, Ein remembered who he was and left for the Mugen Tenshin Ninja Clan.

In the second tournament, Ein meets Helena Douglas who confused the amnesiac Ein by talking to him about the Epsilon Project, but he didn't remember anything about it. In the end, they fight and Helena was defeated by him.

After the second tournament, Ein finally meets up with Kasumi, Hayate's younger sister and, following Hayabusa's advice, they battled each other. Although Kasumi was defeated, Hayate finally gained back his memories.

During the second tournament, Ein fought Hayabusa and started to remember who the ninja is. When he met Kasumi, he followed Hayabusa's advice to battle Kasumi; after defeating her, Hayate finally gained back his memories.

When Hayate was undergoing Project Epsilon, Lisa was one of the scientists connected to the project and possibly the lead. When Ryu had ambushed DOATEC in an effort to free Kasumi, Lisa fled, grabbing Kasumi Alpha and Hayate (who was still without his memories at that time) and it's presumed that she somehow freed him and left him in a forest in Germany where Hitomi meets him. In Dead or Alive 4, with his memories restored, he engages La Mariposa in combat as a way to "thank her" for freeing him, possibly meaning he's aware of her identity when she's being her alter ego.

In Dead or Alive 5 Ultimate they share a tag intro, where Lisa says that it's up to her to stop him, but Ein ignores her claiming no one can stop him. They also share a tag win pose where Ein goes into his signature strike stance while Lisa strikes an elegant pose to commemorate their victory.

Being the predecessor to Hitomi, Ein is slower but more powerful. While Hitomi is better for an aggressive play style, Ein is better for playing defensively and spacing from the opponent, due to his lack of strings, slower speed, his standing and crouching offensive holds, and his more numerable tools to keep the opponent away. Compared to Hitomi, he can exploit environmental stages more easily than her as he has more moves that send the opponent flying across the stage, such as his various "power punches" and his low kick hold (Kobo). Such quick, high-damage strikes also make him particularly suited to Time Attack mode. He has a smaller moveset than Hitomi, and he is also easy to pick up. However, he has one particular move, the Nobori-Mawashi-Geri, his strongest launcher and an essential move to make his most damaging combos, which needs a bit of practice to pull off quickly and consistently, adding some technicality to him. It is also noted that a lot of his moves are unsafe on block.

Ein is an unlockable character in Dead or Alive 3 and Dead or Alive 4, in which he can be used in every gameplay mode except Story Mode. To unlock him:

Dead or Alive 3: Finish Story Mode with every character then complete Survival Mode with Hayate, getting enough victories so you can put your name in the rankings. While at the ranking screen, enter EIN as the name.